1. Field of the Invention
The present invention relates to a foreign substance inspection apparatus, which efficiently detects foreign substances on a reticle called a photomask or on a protective film called a pellicle, an exposure apparatus having the inspection apparatus, and a method of manufacturing a device. The reticle has a circuit pattern formed on it, which is used to manufacture a device, for example, a semiconductor device such as an integrated circuit or a large-scale integration, a charge-coupled device, a liquid crystal panel, or a magnetic head. The pellicle is a protective cover attached to the reticle in order to prevent any foreign substances from adhering onto the reticle.
2. Description of the Related Art
In the process of manufacturing a device, an exposure apparatus (a stepper or a mask aligner) generally transfers a circuit pattern formed on a reticle onto a wafer coated with a resist. During this transfer, if a pattern defect or a foreign substance such as dirt is present on the reticle, the pattern defect or the foreign substance is transferred onto the wafer, together with the circuit pattern, which lowers the device manufacturing yield. Especially when a large number of circuit patterns are repeatedly printed on a wafer by the step & repeat method using a reticle, one harmful foreign substance, if any, on the reticle is printed on the entire wafer, and this considerably lowers the device yield. Hence, in the process of manufacturing a device, it is necessary to detect the presence of foreign substances on the reticle. An inspection method which uses a property that a foreign substance isotropically scatters light is commonly adopted for this detection. For example, some schemes inspect a surface to be inspected by obliquely applying a parallel light beam onto the surface to be inspected from above, and guiding the light scattered by a foreign substance onto a one-dimensional image sensor (to be referred to as a “line sensor” hereinafter) in a gradient index microlens array to image the foreign substance on the sensor. Japanese Patent Laid-Open Nos. 7-43312 and 7-5115 disclose details of these schemes.
To convert the detected intensity of scattered light into the particle size of a foreign substance, the conventional foreign substance inspection apparatus uses a histogram of a detection signal obtained upon inspecting a test reticle coated with reference particles. A plurality of test reticles are used for respective types of reference particles with different sizes (e.g., 10 μm, 20 μm, 30 μm, and 40 μm). The inspection apparatus inspects the test reticles for foreign substances, and calculates the frequency distributions (to be referred to as “histograms” hereinafter) of the detection voltages. The inspection apparatus then reads most representative detection voltages from the histograms for respective types of reference particles, and determines a conversion table to convert the detection voltage into the particle size of a foreign substance. The inspection apparatus also interpolates using discrete data of the conversion table to obtain a conversion curve.
However, the conventional inspection apparatus uses a table or curve common to the entire region on an inspection surface based on histograms, so it can obtain only one average conversion table or conversion curve for each inspection surface. Thus, unless the intensity of scattered light and the particle size of a foreign substance, detected by the inspection apparatus, have a uniform relationship within the plane of the inspection surface, the exact particle size of the foreign substance cannot be obtained.